1. Field of the Invention
The present invention relates to a fuel cell stack including a cell assembly of unit cells stacked together, and a method of supplying reactant gases to the fuel cell stack. Each of the unit cells has a membrane electrode assembly including an anode, a cathode, and an ion exchange membrane of solid polymer electrolyte interposed between the anode and the cathode.
2. Description of the Related Art
Generally, a solid polymer electrolyte fuel cell employs a membrane electrode assembly (MEA) which comprises two electrodes (anode and cathode) and an electrolyte membrane interposed between the electrodes. The electrolyte membrane is a polymer ion exchange membrane (proton exchange membrane). Each of the electrodes comprises a catalyst and a porous carbon sheet. The membrane electrode assembly is interposed between separators (bipolar plates). The membrane electrode assembly and the separators make up a unit of the fuel cell (unit cell) for generating electricity. A plurality of unit cells are connected together to form a fuel cell stack.
In the fuel cell, a fuel gas such as a hydrogen-containing gas is supplied to the anode. The catalyst of the anode induces a chemical reaction of the fuel gas to split the hydrogen molecule into hydrogen ions (protons) and electrons. The hydrogen ions move toward the cathode through the electrolyte, and the electrons flow through an external circuit to the cathode, creating a DC electric current. An oxygen-containing gas or air is supplied to the cathode. At the cathode, the hydrogen ions from the anode combine with the electrons and oxygen to produce water.
In the fuel cell, if the ion exchange membrane is dried, it is not possible to keep the operation of high output density. Therefore, it is desirable to moisten the ion exchange membrane suitably. Further, the water produced in the electrochemical reaction should be discharged from the fuel cell stack, particularly from the cathode in order to prevent the voltage drop of unit cell due to the condensation of water vapor, for example.
In an attempt to provide a solution for the problems, for example, Japanese laid-open patent publication No. 11-312531 (prior art) discloses a fuel cell device which includes cell stacks connected in series. The cell stack includes a plurality of unit cells stacked together. Each of the unit cells includes separators and a membrane electrode assembly interposed between the separators. The membrane electrode assembly includes an anode, a cathode, and an ion exchange membrane of solid polymer electrolyte interposed between the anode and the cathode. Reactant gases flow through the cell stacks operated at low, middle, and high temperatures. The temperature of cell stacks increases in the flow direction of the reactant gases. The reactant gases are humidified according to the operating temperature of the cell stack flowing at first, and supplied to the cell stacks.
In the prior art, the fuel cell stacks are operated at the low, middle, and high temperatures, such that the temperature of the fuel cell stack increases in the flow direction of reactant gases. Therefore, even if water vapor is condensed in the fuel cell stack operated at the low temperature, the condensed water is vaporized in the fuel cell stack operated at the middle or high temperature. Therefore, reaction of the oxygen-containing gas is carried out suitably.
In the prior art, the fuel cell stacks need to be controlled at different temperatures, for example, at 50°, 60°, 65°. Therefore, the temperature control is difficult, and a complicated device is needed for the temperature control.